various noble metal catalysts, such as Au, Ag, and Pd have demonstrated impressive potential for yielding CO through CO 2 RR. [2] Furthermore, Ir/Au [1a] and 3D Pd nanosheets [3] offer sufficient performances for Zn-CO 2 batteries. These electrocatalysts are, however, expensive and often require high overpotentials to generate desired products. In addition, the Faradaic efficiency (FE) for CO 2 RR in Zn-CO 2 batteries, particularly for CO 2-to-CO conversion, is typically below 50% at relatively high discharge currents (e.g., 5 mA cm −2). [4] As an alternative to noble metal catalysts, nitrogen-coordinated single-metal active sites anchored within porous carbon (M-N-C) have been recently identified as a new class of efficient CO 2 RR catalysts to enable CO 2-to-CO conversion, because of their abundance, high electrical conductivity, and good durability. [5] Among various M-N-C catalysts, Fe-N-C presents a low onset potential for CO production, whereas its CO Faradaic efficiency (FE CO) and partial current density (J CO) are not high, particularly for concentrated electrolytes, [6] owing to the strong binding of *CO on the single Fe-N x site. [7] Strategies such as modifying the morphology and pore structure of the carbon substrate, [6a,8] and adjusting the coordination structure as well as the local environment of the metal center, [4b,5f,9] were employed to enhance CO 2 RR performance. In addition to the single Fe-N x sites, nonmetal moieties in the carbon plane of M-N-C, such as N-doped sites and intrinsic defects, may additionally contribute to CO 2 RR (Figure S1, Supporting Information). Pyridinic and hydrogenated nitrogen species in Fe-N-C was demonstrated to exhibit preferential adsorption for CO 2 and acted as active sites for CO 2 RR. [5h,10] Moreover, the intrinsic carbon defects could also act as sole metal-free active sites for CO 2 RR, stemming from the electron redistribution around the defects, and consequently, form partially positive C atoms, as demonstrated very recently in certain studies. [11] For example, defectrich and metal-free mesoporous carbon materials exhibited enhanced CO generation from CO 2 RR, whereas the competing HER was suppressed. When an overpotential of 490 mV was applied, the FE CO was ≈80%, with a J CO of −2.9 mA cm −2. [11b] A positive correlation was also observed between the content of intrinsic carbon defects and the CO 2 RR performance of carbon-based catalysts. [11a] Despite these achievements, the J CO for the reported carbon catalysts containing intrinsic defects remained below 7 mA cm −2. [11] Considering that such catalysts Manipulating the in-plane defects of metal-nitrogen-carbon catalysts to regulate the electroreduction reaction of CO 2 (CO 2 RR) remains a challenging task. Here, it is demonstrated that the activity of the intrinsic carbon defects can be dramatically improved through coupling with single-atom Fe-N 4 sites. The resulting catalyst delivers a maximum CO Faradaic efficiency of 90% and a CO partial current density of 33 mA cm −2 in 0.1 m KHCO 3. The ...
Replacing conventional metal–N4 moieties with different coordination structures is a promising strategy to tailor the activity and selectivity of single-atom catalysts (SACs). However, for CO2 electroreduction driven by metals that may produce diverse chemical species, such as Tin (Sn), the influences of nonnitrogen coordination environments on the CO2 reduction pathways are unclear. Herein, we report an Sn SAC with a special coordination structure of Sn-C2O2F, which delivers CO as an exclusive CO2 reduction product with a faradaic efficiency higher than 90.0% over a wide potential window (−0.2 to −0.6 V vs the reversible hydrogen electrode) and a peak value of up to 95.2%. The resulting cathodic energy efficiency and current density achieve 70.7% and 186 mA cm–2, respectively. In contrast, formate is predominantly formed on the Sn-N4 site. Theoretical calculations disclose that C and O coordination modulates the adsorption of intermediates, while an Sn-bonded F atom significantly suppresses the hydrogen evolution, thereby facilitating CO2-to-CO conversion. Meanwhile, the CO2-to-HCOO– conversion via O-bound intermediates or direct reaction of absorbed H and dissolved CO2 is prohibited.
Carbon-based electrocatalysts with single metal sites hold great potential for mechanism exploration via mimicking molecular catalysts, due to their distinct catalytic sites. In addition to metal atoms, the neighboring nonmetal heteroatoms such as N, S, and O atoms, which are widely detected in carbon-based single-atom catalysts, may also contribute to enhancing the electrochemical activity of singlemetal centers. In this work, the boosting effect of O-doping toward the electrochemical oxygen reduction reaction (ORR) was evaluated by both experimental studies and DFT calculations. O-doped carbon-supported single-Fe-site catalysts possessing deep mesopores and desirable hydrophilic surface were achieved by confined carbonization in an inert or reductive atmosphere (SAFe-NDC and SAFe-NDC-H). As compared to the state-of-the-art Pt/C, these catalysts showed superior catalytic activity toward the ORR in terms of half-wave potential, Tafel slope, and long-term stability. In particular, SAFe-NDC-H outperformed its SAFe-NDC counterpart. Considering that these two catalysts possess a comparable porous structure, surface properties, and local electronic structure of a single Fe site, the dopant nonmetal O atoms, specifically, carbonyl group (CO), are revealed to affect the ORR activity of the single Fe site exclusively. The introduced CO facilitates the formation of *OOH as well as the reduction of *OH, thereby reducing the catalysts' overpotential.
Carbon-based matrix is known to exert a profound influence on the stability and activity of a supported molecular catalyst for electrochemical CO2 reduction reaction (eCO2RR), while regulating the interfacial π–π interaction by designing functional species on the carbon matrix has seldom been explored. Herein, promoted π electron transfer between a graphene substrate and cobalt phthalocyanine (CoPc) is achieved by introducing abundant intrinsic defects into graphene (DrGO), which not only generates more electrochemically active Co sites and leads to a positive shift of the Co2+/Co+ reduction potential but also enhances the CO2 chemical adsorption. Consequently, as compared to the defect-free counterpart rGO-CoPc, DrGO-CoPc could yield CO with a Faradaic efficiency (FECO) higher than 85% in a wide potential range from −0.53 to −0.88 V, and the largest FECO and partial CO current density (J CO) achieve 90.2% and 73.9 mA cm–2, respectively. More importantly, both FECO and J CO can be dramatically improved when conducting eCO2RR in an ionic liquid-based electrolyte, for which FECO is higher than 90.0% in a wide potential range of 600 mV, with the peak J CO of up to 113.6 mA cm–2 in an H-type cell. The excellent eCO2RR performance of DrGO-CoPc rates itself as one of the best immobilized molecular catalysts.
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